This invention relates to adhesive compositions, and more specifically to moisture-curable adhesives useful for bonding substrates such as, inter alia, metal surfaces. In particular, the invention relates to adhesive combinations of acrylate and epoxy.
A long-standing problem in the field of moisture-curable adhesives is the inability to provide adequate initial adhesion of two substrates within a short time period. This problem is particularly serious in industrial applications which do not allow sufficient time for adequate adhesion to develop. A common example is in assembly lines, where an initial bond must be developed prior to performing the next assembly operation. In automotive assembly and other industrial activities in which relatively large pieces of metal are adhesively bonded, there is the added requirement that the newly-applied adhesive bond withstand the various stresses associated with manipulation and handling of heavy metal subassemblies.
Adhesives developing high strength after prolonged or high temperature cure usually provide inadequate initial adhesion in the relatively short time periods of assembly line operations. A particular application in which good initial adhesion is critical is the bonding of metal substrates prior to post-curing of the bond at elevated temperatures. Without high initial adhesion, the bond will not hold during the heat-up to the higher temperatures of the post-cure operation.
The ability to develop adhesion is tied to the rate of cure of the polymer in the adhesive. As the molecular weight of the polymer increases due to partial curing, the polymer develops tack, i.e., a degree of adhesion. The term "green strength" is used to characterize partial cure by polymerization which occurs at ambient temperatures. An adhesive with high green strength thus has high initial adhesion or tack and desirably can maintain adhesion even upon exposure to elevated temperatures.
Acrylic is one type of adhesive useful in bonding a metal substrate either to another metal substrate or to a non-metal substrate. Acrylic-based adhesives can be made to provide a level of green strength by admixture with latent amine curatives, thereby providing rapid moisture cure at ambient temperatures. However, the strength of bonds using only ambient temperature moisture cure is insufficient for anything other than initial adhesion, being useful as a temporary or initial bond but inadequate for permanent adhesion. Subsequent post-curing of acrylic adhesives generally does not significantly add strength beyond the initial adhesion level. Thus, while acrylic adhesives may have the green strength and initial adhesion needed for assembly line operations, by themselves they are insufficient to provide a permanent bond for end product use.
Epoxy resins, on the other hand, provide very good adhesion even for metal-to-metal bonding after curing at high temperatures. However, admixture with latent amine curatives and exposure to moisture requires objectionably long times before development of any green strength. Hence unmodified epoxies are unsuitable for many applications requiring green strength.
The use of epoxy and acrylate with ketimines is known, as exemplified by U.S. Pat. No. 4,616,066. However, this patent relates to epoxy resins which are modified by vinyl or acrylic polymers, and in which no unpolymerized acrylic functions remain to participate in the subsequent curing reaction.
Epoxy blends with ketimines are disclosed in U.S. Pat. No. 4,861,832. Optional use of a multifunctional acrylate monomer is disclosed at Col. 2, line 37 to Col. 3, line 9. The acrylate monomer is combined with the epoxy resin to form a first component of a two-component epoxy resin composition, the second component being a hardener for curing the epoxy resin. The hardener is a mixture of a ketimine and an amine-terminated butadiene/acrylonitrile copolymer rubber. The use of this hardener is alleged to provide rapid cure of the epoxy. That patent does not teach that acrylate and epoxy blends can effect a two-stage cure. To the contrary, the goal of rapid epoxy cure suggests that the blends of U.S. Pat. No. 4,861,832 are cured in a single stage. Furthermore, the patent is restricted to two-component systems with limited pot life once mixed.
It is also known in the prior art to use ketimines as curing agents either alone or in combination with other amine-terminated curing agents. Ketimines function as moisture-actuated latent curing agents both for epoxy resins and for acrylate compositions. The typical commercial ketimines are difunctional.
In many applications such difunctional ketimines perform satisfactorily. When formulated with epoxy resins, however, commercial diketimines based upon methyl isobutyl ketone (MIBK) are unstable because the ketimine readily isomerizes to the enamine form, which can react with the epoxy. The mechanism of conversion to enamine is disclosed in U.S. Pat. No. 3,547,886. This patent discloses ketimines made from diisopropyl ketone (DIPK) as a solution to the problem.
Another problem with prior art ketimines is that, where a very rapid moisture cure at ambient temperatures is desired, the rate of cure using difunctional ketimines is generally inadequate.
Higher functional ketimines, i.e., those having a functionality of greater than 2, are also known. U.S. Pat. No. 3,993,708 discloses di- and trifunctional ketimines for moisture cure of epoxy. This patent does not disclose the use of acrylate/epoxy blends as adhesives. U.S. Pat. No. 4,096,105 discloses a tetrafunctional ketimine made by coupling diethylenetriamine bis(methylether ketimine) with a diepoxide. The ketimine is acidified, mixed with acrylate monomers, dispersed in water and applied as a coating. Acrylate/epoxy blends are not disclosed. U.S. Pat. No. 4,391,958 discloses triketimines used for curing urethane or epoxy systems, but acrylates are not included. The alleged advantages of using trifunctional ketimines over difunctional ketimines are improved stability of one component formulations and improved transparency of cured film.
The higher functional ketimines of these patents are not claimed to provide significant increases in rate of reaction compared to the more common diketimines. Moreover, these higher functional ketimines are susceptible to isomerization to the corresponding enamine form, thereby shortening shelf life.
The present invention solves the problem of providing adequate initial adhesion via green strength, while simultaneously providing a strong permanent adhesive bond for end product use. Furthermore, the invention solves the problem of stability associated with prior art curable adhesive compositions containing ketimines as a curing agent.